The present invention generally relates to control pulse recording circuits for magnetic recording and reproducing apparatuses, and more particularly to a control pulse recording circuit for recording control pulses in a double-gap system magnetic recording and reproducing apparatus.
A double-gap system magnetic recording and reproducing apparatus comprises two pairs of video heads, where each pair is constituted by video heads having gaps of mutually different azimuth angles. The two pairs of video heads are selectively used depending on a moving speed of a magnetic tape so as to record and reproduce a video signal on and from the magnetic tape. In the present specification, the video heads which are used in a standard play mode will be referred to as standard play heads, and the video heads which are used in an expanded play mode will be referred to as expanded play heads. For example, in the case where the magnetic tape is originally intended to provide two hours of play, it is possible to obtain two hours of play in the standard play mode and obtain six hours of play in the expanded play mode. When the relative height positions of the standard and expanded play heads from a reference height position are close the ideal relative height positions, it is necessary to shift the recording timing of the control pulses by one field (16.7 msec) with respect to rotational phase detection pulses between the standard and expanded play modes. The rotational phase detection pulses are obtained by detecting the rotational phase of a drum which is mounted with the video heads. Hence, in a conventional control pulse recording circuit, a signal which is used to form the control pulses for the standard play mode is delayed in a delay circuit so as to form the control pulses for the expanded play mode. For this reason, there are problems in that it is impossible to simplify the circuit construction of the control pulse recording circuit and the circuit is expensive.
An actual difference C1 between the relative height positions of the standard and expanded play heads does not become equal to an ideal difference C0 between the ideal relative height positions of the standard and expanded play heads. In other words, a difference P=C0-C1 exists between the actual difference C1 and the ideal difference C0 with which an X-value becomes a designed value. The X-value is a distance on the magnetic tape from a control head to a video head which terminates a scan on the magnetic tape. For this reason, it is necessary to adjust the X-value in the standard and expanded play modes.
Conventionally, the position of the control head is adjusted so that the X-value becomes a regular value in the standard play mode. On the other hand, in the expanded play mode, an error in the X-value caused by the actual difference C1 is corrected electrically. That is, the position of the control head is adjusted so that the X-value becomes the regular value in the standard play mode, and the conventional circuit records the control pulses with the timing of the vertical synchronizing signal. Furthermore, in the expanded play mode, the conventional circuit records the control pulses with a timing delayed with respect to the timing of the vertical synchronizing signal, by use of a delay circuit. The delay circuit has a delay time of 29 msec which is relatively long, and is easily affected by inconsistencies in the temperature characteristic and the like thereof. In addition, there are problems in that a variable resistor is required to adjust the delay circuit and that the setting of the control pulse recording circuit must be changed between the standard and expanded play modes.